A great variety of concealed weapon detection (CWD) systems have been suggested. These systems exploit different physical principles such as 1) electromagnetic-wave radiation in the radio-frequency (RF), microwave and millimeter-wave frequency bands; 2) detection of distortions in a background magnetic field (magnetic systems); 3) magnetic resonance imaging (MRI); 4) inductive magnetic field methods; 5) acoustic and ultrasonic detection; 6) infrared imagers; and 7) X-ray imagers.
Systems employing electromagnetic-wave radiation are often separated into two distinct classes of CWD systems: imagers and detectors. Imagers generate an image of the inspected target and the image is inspected (usually by a human operator) for suspicious objects. Some such imaging systems are already available commercially, e.g. the “whole-body scanners” in operation at many major airports around the world.
Whole-body scanners operate in and around the millimeter-wave frequency ranges (typical operational frequencies lie between 30 GHz and 300 GHz). These systems are often expensive and bulky. These systems also require the full cooperation of the inspected person. The inspected person must not only stay still but also have both arms raised and legs spread apart to allow the millimeter waves to illuminate the whole body surface. Whole-body scanners may also raise privacy concerns because they produce images with fairly detailed outline of the human body.
A number of previous attempts have been made to provide a useful weapons detection system. Examples of previously suggested systems include those described in U.S. Pat. No. 6,342,696 entitled “Object Detection Method and Apparatus Employing Polarized Radiation” to Chadwick; U.S. Pat. No. 6,359,582 (Canadian Pat. No. 2,265,457) entitled “Concealed Weapons Detection System” to MacAleese at al.; U.S. Pat. No. 6,831,590 entitled “Concealed Object Detection” to Steinway et al.; and U.S. Pat. No. 7,518,542 entitled “Handheld Radar Frequency Scanner for Concealed Object Detection” to Steinway et al. These systems all generally require an inspected person's cooperation for effective use. Many such systems, such as microwave detection systems, also tend to have insufficient detection reliability.